This application claims the priority of German Patent Application No. 198 20 828.6, filed May 9, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust gas cleaning system, having (1) a first catalyst unit which, in the case of a rich exhaust gas composition, generates ammonia from corresponding exhaust gas constituents; and (2) a second catalyst unit connected behind the first which, in the case of a rich exhaust gas composition, intermediately stores ammonia generated by the first catalyst unit. In the case of a lean exhaust gas composition, the second catalyst unit subjects nitrogen oxides (NO.sub.x) contained in the exhaust gas to a reducing reaction using the intermediately stored ammonia as a reducing agent.
The term "rich exhaust gas composition" relates to the composition of the exhaust gas that is obtained during rich operation (with a fuel fraction above the stoichiometric ratio) of the combustion process. The term "lean exhaust gas composition" relates to the composition of the exhaust gas that is obtained during lean operation (with a fuel fraction below the stoichiometric ratio) of the combustion process. Such an exhaust gas cleaning system can be used, for example, for a motor vehicle engine in order to minimize its nitrogen oxide emissions.
For achieving low nitrogen oxide emissions in motor vehicle combustion engines, for example, it is known to drive the engine alternately in rich and in lean engine operation and to provide an exhaust gas cleaning system having one or several parallel nitrogen-oxide adsorber catalysts. The nitrogen-oxide adsorber catalysts adsorb nitrogen oxides in lean engine operating phases and desorb them in rich engine operating phases, thereby reducing them by means of exhaust gas recirculation or another technique.
Furthermore, exhaust gas cleaning systems are known in which a nitrogen oxide reducing agent is added from the outside in order to reduce nitrogen oxides contained in the fed exhaust gas to nitrogen.
An exhaust gas cleaning system of the initially mentioned type is described in European Published Patent Application EP 0 15 773 354 A1 for cleaning the exhaust gas of an internal-combustion engine. The ammonia-generating catalyst unit is a three-way catalyst which, during a rich engine operation, synthesizes ammonia from nitrogen oxides and hydrogen which are contained in the exhaust gas. The generated ammonia is adsorbed in the second catalyst unit connected behind the first and is intermediately stored. In the intervals with the lean internal-combustion engine operation, the three-way catalyst lets the nitrogen oxides contained in the exhaust gas pass to the second catalyst unit connected on the output side, where the ammonia is then released and is used as a reducing agent for reducing the nitrogen oxides. The nitrogen oxides are converted to nitrogen with simultaneous oxidation of the ammonia. The two catalyst units contain suitable catalyst materials for fulfilling the above-mentioned functions: for example, palladium (Pd) or cerium (Ce) for the three-way catalyst, and a zeolite, silicon oxide, aluminum oxide and/or titanium oxide material with copper (Cu), iron (Fe), platinum (Pt), palladium (Pd) and/or rhodium (Rh) for the ammonia-adsorbing and oxidizing catalyst unit. As soon as the intermediately stored ammonia quantity is exhausted in the lean operation, a switch-over takes place to a rich internalcombustion engine operation.
One difficulty of this system is that, in the rich operation, significantly fewer nitrogen oxides are contained in the exhaust gas and therefore a correspondingly low amount of ammonia is produced. The rich operation, which is unfavorable with respect to the fuel consumption, must be maintained for a relatively long time in order to obtain a defined significant amount of ammonia.
The present invention is based on the technical problem of providing an exhaust gas purification system by means of which the nitrogen oxide content of combustion exhaust gases, such as motor vehicle engine emissions, can be reduced effectively and at comparatively low expenditures and the corresponding combustion process can be carried out with lean-operation which is as high as possible.
The present invention solves this problem by providing an exhaust gas cleaning system having the following characteristics.
A first catalyst unit is connected in front of a second catalyst unit. The second catalyst unit, in the case of a rich exhaust gas composition, produces ammonia from corresponding exhaust gas constituents. In a lean exhaust gas composition, the first catalyst unit intermediately adsorbs and stores the nitrogen oxides contained in the lean exhaust gas and desorbs them in a rich exhaust gas composition. The first catalyst unit can be formed, for example, by a conventional nitrogen-oxide adsorber catalyst. In the case of a lean exhaust gas composition (in the lean operation of a corresponding internal-combustion process in which more nitrogen oxides occur than in a rich internal-combustion process), a comparatively large amount of nitrogen oxide can be intermediately stored in the first catalyst unit.
When the first catalyst unit is saturated with intermediately stored nitrogen oxides, during a further lean operation, the nitrogen oxides contained in the exhaust gas flow through the ammonia-producing catalyst unit, which essentially remains passive in the lean operation, to a third catalyst unit which intermediately stores ammonia. In the third catalyst unit the nitrogen oxides are reduced to nitrogen while the intermediately stored ammonia is released and acts as a reducing agent. In the case of a rich exhaust gas composition (an internal-combustion process carried out by means of an overstoichiometric fuel-rich mixture), the nitrogen oxides intermediately stored during the preceding lean operation in the upstream first catalyst unit are released and are used in the second catalyst unit for the production of ammonia which is then intermediately stored in the downstream third catalyst unit.
By using the intermediately stored nitrogen oxides, which, in the lean operation occur in much larger quantities than in the rich operation, a correspondingly large amount of ammonia is produced and intermediately stored in a rich combustion process. The stored ammonia is then available for the effective nitrogen oxide reduction in a subsequent lean operating phase. On the whole, this permits an operation with a high lean-operation part and therefore with a correspondingly low fuel consumption.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.